The present invention relates to a contact member for supplying electrical current to a wire electrode on a machine for electro-erosion.
On an electro-erosion cutting machine using an electrode wire, the latter runs continuously during the machining operation and, therefore, it is necessary to supply the machining current, as well as an eventual auxiliary current by means of a contact with respect to which the wire is moved. For that purpose, it is usual to slide the wire on a friction member, generally having a convex surface, for example a fixed device, having the shape of a pulley or finger or a rotating cylinder around a fixed axis, the latter being made of tungsten, preferably, or other friction and heat resistant material and connected to a current generator. The relatively high traction exercised on the wire during its passage in the machining zone causes the wire to exert a strong pressure against the friction member, resulting in a fast wear of said friction member located at the running point of the wire, and requires it to be replaced periodically, even if it is made of a very strong abrasion-resistant material. Furthermore, the dry friction of the wire on the friction member causes an effect on the wire tension behind the friction member which can be hardly controlled because it depends, among other, on the amount of wear. Further, the intensity of the supply current, and since it is established generally by a single tangential surface, necessitates a strong wire pressure against the friction member for the purpose of improving the contact.
Because of the friction it creates, the wire deposits on the friction member a layer of material of which the wire is made, for example, copper, cadmium, tungsten or molybdenum or alloy oxide of these materials as, for example, brass. This creates micro-welds between the wire and the friction member. The thin layer is also due to the micro-discharges releasing enough heat to cause local melting of the material which forms the surface of the wire. The micro-welds interfere with the proper operation of the wire and impede the regulating of the machining current. Furthermore, the wire friction on the contact member digs a groove in the latter. It has also been observed that when the depth of the groove is equal to the radius of the wire, the risk of wire breakage increases considerably. It becomes then necessary to use another part of the friction member or to swap it. Furthermore, because of the wire abrasion, powder or chips appear at the site. The relatively high pressure of the wire against the friction member tends also to deform the wire, which interferes with high precision machining.
The present invention aims to eliminate these defects and to create an excellent electrical contact between the wire and the electrical supply source. For that purpose, the contact member, according to the present invention, is characterized by a conducting member designed as a freely rotating part which is attached solidly to a shaft arranged perpendicularly to the plane of rotation. This shaft is preferably made to be conducting and extends into a cavity containing a quantity of conducting fluid such as a paste or liquid, establishing electrical contact between the shaft and the conducting internal surface, of the cavity.
It is advantageous to use a metal roller as the conducting part, which can have a V-shaped groove on its periphery.
The present invention substantially eliminates all friction, and therefore, increases the longevity of the contact member and makes it possible to provide the wire with large currents without creating hot points. For example, when the friction members are made of tungsten, as in the present state of the art, they must be replaced in about 30 hours of operation; a member made according to the present invention, when made of bronze, for example, can be used at least for approximately 600 hours permitting the establishment of an automatic procedure for electroerosion machining. If the conducting part is made of tungsten, the utilization period can be even longer.
Another advantage introduced by the substantial elimination of friction is that it is possible to fabricate the conducting part of a material which does not necessarily have a great resistance to abrasion, and therefore which is less expensive than tungsten, such as an alloy as bronze or stainless steel.
Another additional advantage is substantially the absence of abrasion of the electrode wire which remains practically intact. It does not create tracks due to friction. Furthermore, there is no presence of powder or chips due to abrasion.
The disappearance of hot spots reduces the wear of the electrode wire and the risk of breakage of the latter.
With the contact members of the present invention, it is now possible to increase greatly the amount of current flowing through the wire electrode without causing the breakage of the latter.
Furthermore, in most cases, due to the disappearance of hot spots, it is possible to eliminate contact lubrication altogether.
The contact member of the present invention assures a good contact with the conducting wire, because the contacting part and its shaft are driven together during their rotation. The current flow passing from the shaft into the contacting part is not made intermittent becaus of the play of the shaft in the rotating part, as it was the case for certain friction members used in the present state of the art.
The quality of the contact is further improved in the case of the mercury contact utilized in one of the preferred form of execution used in the present invention. This type of contact insures a permanent passage of current between the shaft and the body of the contact member connected to the generator, because the contact is independent of the rotation of the shaft. Furthermore, the movement of the latter does not suffer from the braking effect caused by a friction part supplying the current to one of the rounded ends of the shaft, generally made of carbon, and held in position by a spring against the rounded end. The current supply by mercury contact provides many other advantages over the supply using a carbon, for example. In particular, the contact surface is obviously larger, because it is not limited to the area of a rounded tip, since the end of the shaft is completely submerged on a given surface. There is no need to take in account the wear of the carbon (very fast) which gives a real autonomy to the assembly, since there is no need to replace any of the parts for many hours of operation (several days).
Because of the excellent quality of the contact obtained with contact member of the present invention, it is now possible to reduce the wire pressure on the contact member and, therefore to reduce the plastic deformation of the wire. Furthermore, since it is knowm from the present state of the art, that this wire pressure is determined by the curvature of the wire, with the present invention, it is now possible to reduce said curvature. This reduces the wire pressure on the auxiliary guides made of sapphire, located in front of the upper contact and behind the down contact. This results in a substantial reduction of the wire abrasion produced on the sapphire surfaces of these guides.
The loss of electrical energy is enormously reduced. Furthermore, because of the minimal size of the contact member (approximately 12 mm in diameter for the conducting part and 1 mm.sup.3 of mercury is sufficient to provide an excellent contact and, in certain cases, there is no need to have a lubricant supply system) it is now possible to install it in the immediate proximity of the machining area which further reduces the losses of energy. Therefore, an important improvement of the machining current control is obtained.
All of these advantages are obtained without introducing any inconvenience with respect to the contact members used in the present state of the art. In particular, the size of the contact members, according to the present invention is as small as any of the known friction members. It must be added that they are also orientable and work in any direction.